Method for activating a semiconductor electron emitter

ABSTRACT

THE METHOD FOR ACTIVATING A SEMICONDUCTOR TO MAXIMIZE ITS ELECTRON EMMISSION, ESPECIALLY PHOTOEMISSION, INCLUDES THE STEPS OF SENSITIZING A CLEANDED SEMICONDUCTOR WITH A LEAST A STRONGLY ELECTRONEGATIVE MATERIAL AND A STRONGLY ELECTROPOSITIVE MATERIAL, THEREAFTER HEATING THE SEMICONDUCTOR TO A TEMPERATURE OF BETWEEN ABOUT 470*C. $10*C., AND 590*C$10*C., THEN RESENSITIZING THE SEMICONDUCTOR WITH AT LEAST A STRONGLY ELECTROPOSITIVE MATERIAL.

June 13, 1972 D. G. FISHER METHOD FOR ACTIVATING A SEMICONDUCTOR ELECTRON EMITTER Filed Sept. 4, 1970 III) SENSITIZTNG THE SURFACE WITH CESIUM AND OXYGEN HEATING TO ABOUT 535 C.

T COOLING TO ABOUT 25 C.

RE-SENSTTTZING THE SURFACE WTTH CESIUM AND OXYGEN Fig. 2.

I N VEN TOR. Dennis 6 Fisher By 140% RUM'JJ.

ATTORNEY United States Patent O 3,669,735 METHOD FOR ACTIVATING A SEMICONDUCTOR ELECTRON EMITIER Dennis Glendon Fisher, Princeton, N.J., assignor to RCA Corporation Filed Sept. 4, 1970, Ser. No. 69,537 Int. Cl. H011 7/32, 7/44 U.S. Cl. 117224 10 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION The invention disclosed herein was made in the course of, or under, a contract or subcontract thereunder with the Department of the Army.

The invention relates generally to a method for increasing the electron emission into vacuum of semiconductor surfaces, and is particularly applicable where the semiconductor is a III-V compound.

Semiconductor compounds containing one or more elements from each of Groups III-A and V-A of the Periodic Table of the elements have been found to be particularly useful for emitting electrons intovacuum. The electron emission efficiency of such emitters is especially high, since surfaces of these semiconductors can be activated with a strongly electronegative and a strongly electro-positive material such as oxygen and cesium, respectively, to produce a negative effective electron afiinity. Semiconductor electron emitters and methods of activating them by sensitizing with low work function materials to increase photoemission are discussed, for example, in the following references:

US. Pat. 2,770,561 to A. H. Sommer, filed Mar. 8, 1954;

issued Nov. 13, 1956 US. Pat. 3,387,161 to J. Van Laar et al., filed Nov. 18,

1965; issued June 4, 1968 US. Pat. 3,478,213 to R. E. Simon et al., filed Sept. 5,

1967; issued Nov. 11, 1969 Uebbing, J. S. et al., Improved Photoemitters Using GaAs and InGaAs. In Proceedings of the IEEE, 56; p. 1624-5, September 1968 Syms, C. H. A., Gallium Arsenide Thin-Flm Photo cathodes. In Advances in Electronics and Electron Optics. N.Y., Academic Press, 1969, vol. 28A; pp. 399-407 edited by: J. D. McGee, D. McMullan, E. Kahan, B. L. Morgan Schagen, P. et al., New Approches to Photoemission at Long Wavelengths. In Advances in Electronics and Electron Physics. N.Y. Academic Press, 1969, vol. 28A, pp. 3938.

A present method of activating a IIIV compound sample with cesium and oxygen includes first cleaning the sample of adsorbed gases and other contaminants by heating it in vacuum for about one minute to a temperature just below its decomposition temperature of about 650 C.700 C. The sample is then cooled to room temperature and sensitized by deposition of cesium and oxygen.

Sensitizing lowers the work function for electrons near the sample surface. The photosensitivity of the sample is 3,669,735 Patented June 13, 1972 "ice monitored as an indication of the general electron emission performance. In some instances, optimum activation is achieved by resensitizing after each of one or more additional beatings at successively higher temperatures.

The word sensitizing as used here refers to any of a number of various known techniques for increasing electron emission from a surface by depositing other materials on the surface to lower the work function for electrons at the surface. For example, where cesium and oxygen are used, cesium may be continually admitted to the chamber containing the sample, with the oxygen introduced intermittantly, as in the preferred embodiment. When the cesium source can be readily controlled, oxygen may be continually admitted to the chamber, withthe cesium introduced intermittantly. A single introduction of both cesium and oxygen may also be sufficient in some instances.

SUMMARY OF THE INVENTION The novel method for activating a semiconductor electron emitter comprises sensitizing a surface of the semiconductor with at least a strongly electronegative material and a strongly electropositive material, then heating the semiconductor to between about 470 0:10 C. and 590 C.il0 C., and cooling, and then resensitizing the surface with at least a strongly electropositive material.

The novel method results in substantially increased electron emission, especially in III-V semiconductor compounds.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partially sectional, partially schematic view of an apparatus for activating a semiconductor in accordance with a preferred embodiment of the novel method.

FIG. 2 is a flow chart of the preferred embodiment of the novel method as practiced with the apparatus of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the preferred embodiment of the novel method, referring now to FIG. 1, a layer sample 10 of indium gallium arsenide (In Ga As) about 1 cm. square and about 25 microns thick is epitaxially grown on one face of a 20 mils thick gallium arsenide crystal substrate 12. The substrate 12 with the sample 10 is mounted in an ultra-high vacuum chamber 14 connected to a sputter-ion vacuum pump 16 with a pumping speed on the order of 500 liters per second at the chamber entrance. The pump 16 is operated continuously during the entire activation to maintain the sample 10 in a relatively high vacuum as measured by an ionization gauge. A loop of resistance heating wire 18 is mounted near the face of the substrate 12 which is opposite the sample 10. Extending through the wall of the chamber 14 is a closed tube 20 containing at its extended end a small amount 21 of cesium metal. The tube 20 is normally immersed in a flask 22 of liquid nitrogen to prevent unwanted cesium from vaporizing into the vacuum system. Also extending through the wall of the chamber 14 is a closed tube 24 of pure silver sealed shut at its extended end. About the silver tube 24 is an electrically insulating jacket 26 about which is coiled a resistance heating element 28. When the silver tube 24 is heated by the resistance heating element 28, pure oxygen passes from the atmosphere through the silver tube 24 and into the chamber 14. The wall of the chamber 14 further contains a window 30. A lumen light source 32 located outside the chamber directs light to the sample through the window 30. Photoemission from the sample is collected by an anode 36 near the sample 10 and measured by a current sensing device 34.

In the first step of the novel method as shown in the flow chart of FIG. 2, the sample is heated to a cleaning temperature of about 630 C. for a period of about 1 minute. Thereafter, it is cooled to room temperature of about 25 C. and sensitized with cesium and oxygen generally as follows: A small amount of cesium vapor is introduced into the chamber 14 by removal of the liquid nitrogen flask 24 and heating of the cesium-containing tube 20. When the photoemission of the sample 10 is observed to pass a peak, oxygen is introduced into the system in addition to the cesium by heating the silver tube 24. When the photoemission is observed to increase and to reach a. new peak higher than the first peak, the oxygen is shut off while the addition of cesium is continued until a third peak higher than either the first or the second is observed, and so on alternately, until there is no further significant increase from one peak to the next. Generally only between 10 and cycles result in significant increase from peak to peak.

After the sensitizing step is completed, the sample is heated to a temperature of about 535 C. for about one minute, then cooled back to room temperature and resensitized with cesium and oxygen as in the first sensitizing step above.

The novel method results in a completely unanticipated and substantial increase in photoemission from a semiconductor as compared to present methods. The photoemission in terms of microamperes per lumen of emission from some samples, such as indium gallium arsenide, may be as much as twice that of a similar sample activated by present methods. The following chart shows a comparison of the photoemission of samples processed first by present methods and then by the novel method. The column labeled A indicates the maximum photoemissions in microamperes per lumen from separate samples which have been sensitized with cesium and oxygen either after a single heating step or after each of one or more additional heatings at successively higher temperatures. The column labeled "B indicates the maximum photoemission in microamperes per lumen from the same samples, but which have in accordance with the novel method undergone a lower temperature heating step followed by a resensitizing with cesium and oxygen.

GENERAL CONSIDERATIONS While in the preferred embodiments the sensitizing of the semiconductor surface is with cesium and oxygen, a strongly electropositive and a strongly electronegative material, respectively, other combinations of strongly electropositive and strongly electronegative materials are suitable for practicing the novel method. Among the strongly electropositive elements are, for instance, alkali or alkaline earth metals such as cesium and barium. Among the strongly electronegative materials are, for instance, calcogens and halogens such as oxygen and fluorine.

Although the ultra-high vacuum of 10- torr or less pressure is desirable throughout activation, it is not necessarily critical. A lower vacuum, however, may require more extensive cleaning, such as a longer first heating step.

There are various known ways of cleaning a semiconductor sample prior to activation. IHV semiconductors are generally cleaned by heating in a vacuum to at least 600 C. The novel method does not appear to depend on a particular cleaning technique. Other known cleaning techniques which may be used for obtaining a clean semiconductor sample are, for instance, bombardment with an inert gas such as argon and subsequent annealing. Such a. technique is often used with silicon, since silicon doesnot readily clean by baking at high temperatures. For best results, the sensitizing should be done with the temperature of the sample at below 200 C.

In the preferred embodiment, the temperature of the sample was measured with a calibrated infra-red sensitive optical device. Other means, such as a thermocouple, may also be used.

There is no assurance, however, that any of the varioun available temperature measurement means will give the absolute temperature of the sample. The temperature as measured herein is believed to be within about 10 C. of the absolute temperature of the sample. The temperature range of the heating step of the novel method was measured to be from 470 C. l0 C. to 590 C.il0 C. Heating temperatures in the central portion of the range, near 535 C., result in considerably better electron emission from the sample than temperatures near the ends of the range, but temperatures from any portion of the region result in some improvement. The optimum temperature varies somewhat with the time for which the sample is held at that temperature. Lower temperatures generally require longer time.

What is claimed is:

1. A method for activating a semiconductor electron emitter, comprising:

(a) sensitizing a surface of a semiconductor with at least a strongly electronegative material and a strongly electropositive material to substantially increase the electron emission from said surface; then ,(b) heating said semiconductor to a temperature of between about 470 C. and about 590 C.; then,

(c) cooling said semiconductor;

(d) resensitizing said surface with at least a strongly electropositive material, to further increase said electron emission from said surface.

2. The method defined in claim 1 and wherein, prior to said sensitizing, said semiconductor is cleaned of contaminants by heating for about one minute or more to a temperature near, but less than, the decomposition tempera ture of said semiconductor.

3. The method defined in claim 2 and wherein said heating is in a vacuum having a pressure of 10- torr or ess.

4. The method defined in claim 2 and wherein said temperature for cleaning is at least 600 C.

5. The method defined in claim 1 and wherein steps (a), (b), (c), and (d) are carried out while said semiconductor is a vacuum having a pressure on the order of 10- torr or less.

6. The method defined in claim 1 and wherein said sensitizing of step ,(a) is with a strongly electropositive material and a strongly electronegative material until said semiconductor exhibits a substantially maximum electron emission as a result of such sensitizing.

7. The method defined in claim 6 wherein said electropositive material is cesium and said electronegative material is oxygen.

8. The method defined in claim 1 and wherein said second resensitizing of step (d) is with a strongly electropositive material and to a strongly electronegative material until said semiconductor exhibits a substantially maximum electron emission as a result of such sensitizing.

9. The method defined in claim 8, wherein said electropositive material is cesium and said electronegative material is oxygen.

5 10. The method defined in claim 4, wherein said sensitizing is with cesium and oxygen, said heating after said sensitizing is to about 535 C., said cooling is to less than 200 C., and said resensitizing is with cesium and oxygen.

References Cited UNITED STATES PATENTS 3,278,782 10/1966 Kanter 3l394 3,387,161 6/1968 Van Laar et a1. 313-94 6 3,478,213 11/1969 Simon et al 250207 3,549,401 12/1970 Buszko et a1. 117-201 X WILLIAM L. JARVIS, Primary Examiner US. Cl. X.R. 

